Acetic acid produced by the methanol carbonylation process contains oxidizable impurities such as aldehydes (see, e.g., U.S. Pat. No. 7,208,625). A qualitative test, called the Permanganate Time (PT), is used to determine the product quality. The PT test has a Pass or Fail designation. To perform the PT test, a known amount of KMnO4 is added to a known amount of acetic acid. To pass the test, the initial pink color of KMnO4 must not have completely dissipated after two hours. KMnO4, a strong oxidizing agent, can oxidize aldehydes and other oxidizable compounds (also called “permanganate-reducing compound” or PRC). In doing so, MnO4 − is reduced to colorless Mn2+ according to the following reaction:MnO4−+8H+→Mn2++4 H2O
As PT is a visual test, there is considerable operator subjectivity involved in determining the point at which the pink color has disappeared. This visual test is further complicated and compromised by the fact that Mn2+ is relatively unstable and is prone to oxidation by MnO4− to form manganese dioxide (MnO2), a brownish/yellowish species, through the following reaction:2 MnO4−+3 Mn2++2 H2O→5 MnO2+4H+
MnO2 in the reaction mixture interferes with the observation of the pink color. To overcome this difficulty, a recent report discloses a spectrophotometric method for correlating an objective colorimetric measurement with a subjective visual measurement and determining PT based on instrumental analysis (“A New System For Automatic Measurement of Permanganate Time,” Laboratory Automation and Information Management, 34, 1999, 57-67). However, the report does not teach quantitatively determining the PRC content (or concentration) of an acetic acid sample.